This is a fragment of some code I wrote in C which uses MagickWand. It creates a blank 6000x1000 image and then turns on the alpha channel. Perhaps you can find the equivalent of MagickSetImageAlphaChannel in Magick++.

i cant find anything which set an alpha channel in magick++.
the only thing which comes near is image.matte("TRUE").
description from the docu is: True if the image has transparency. If set True, store matte channel if the image has one otherwise create an opaque one.

but this has no effect. also creating a canvas and setting it to "RGBA" and then compositing my image inside has no effect.
any ideas?